Magnetic resonance imaging (MRI) has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities of biological tissue. These images have medical diagnostic value in determining the state of the health of the tissue examined.
Thus, e.g., as is disclosed in U.S. Pat. No. 6,144,205 (the entire disclosure of which is hereby incorporated by reference into this specification), in an MRI process a patient is typically aligned to place the portion of his anatomy to be examined in the imaging volume of the MRI apparatus. Such MRI apparatus typically comprises a primary magnet for supplying a constant magnetic field (Bo) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary magnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). A magnetic field gradient (ΔBz/ΔXi) refers to the variation of the field along the direction parallel to Bo with respect to each of the three principal Cartesian axes, Xi. The apparatus also comprises one or more RF (radiofrequency) coils which provide excitation and detection of the NMR signal.
The use of the MRI process with patients who have implanted medical devices, such as pacemakers often presents problems. As is known to those skilled in the art, implantable devices (such as implantable pulse generators (IPGs) and cardioverter/defibrillator/pacemakers (CDPs) are sensitive to a variety of forms of electromagnetic interference (EMI). These devices include sensing and logic systems that respond to low-level signals from the heart. Because the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, they are vulnerable to external sources of severe electromagnetic noise, and in particular to electromagnetic fields emitted during the magnetic resonance imaging (MRI) procedure. Thus, patients with implantable devices are generally advised not to undergo magnetic resonance imaging (MRI) procedures.
Attempts have been made to protect implantable devices from MRI fields. Thus, for example, U.S. Pat. No. 5,217,010 (to Tsitlik et al.) describes the use of inductive and capacitive filter elements to protect internal circuitry. U.S. Pat. No. 5,968,083 (to Ciciarelli et al.) describes a device adapted to switch between low and high impedance modes of operation in response to EMI insult. U.S. Pat. No. 6,188,926 (to Vock) discloses a control unit for adjusting a cardiac pacing rate of a pacing unit to an interference backup rate when heart activity cannot be sensed due to EMI. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
However, the “solutions” presented by these prior art patents are not entirely adequate. The techniques they describe do not provide a fail-safe system when the protective circuitry or the backup modes of the implantable device fail to protect the implantable device from malfunction due to exposure to electromagnetic fields.
It is an object of this invention to provide a device that will cease furnishing power to a medical device at specified intervals while an individual is undergoing an MRI procedure.
It is another object of this invention to provide a means for furnishing power to a medical device while protecting it from damage induced by certain radio frequency fields.